Hydrocarbon synthesis from hydrogen and carbon monoxide in the presence of a Fischer-Tropsch catalyst is commonly known as Fischer-Tropsch (FT) synthesis. FT synthesis forms part of gas-to-liquids, coal-to-liquids, and biomass-to-liquids processes in which natural gas, coal, and biomass respectively are usually converted by means of a three step process into liquid hydrocarbons. The three process steps are normally (i) production of synthesis gas (or ‘syngas’) comprising a mixture of hydrogen and carbon monoxide from natural gas, coal, or biomass respectively, (ii) conversion of the syngas into a waxy hydrocarbons or syncrude by means of FT synthesis, and (iii) a hydrocracking or hydrotreating step to convert the waxy syncrude into liquid transportation fuels such as diesel, petrol, jet fuel, as well as naphtha.
During the FT synthesis described in step (ii) above the syngas in the form of CO and H2 is contacted with a FT synthesis catalyst under FT synthesis conditions to produce the waxy hydrocarbons. One type of catalyst which is often used in low temperature FT (LTFT) synthesis comprises an active catalyst component such as Co on a catalyst support such as alumina, silica, titania, magnesia or the like.
Contamination of the waxy hydrocarbon product produced during FT synthesis with ultra fine particulate matter derived from the support such as alumina, and the active catalyst component such as Co, is experienced. This results in loss of the expensive active catalyst component as well as fouling of the downstream processes described in (iii) above with the support and active catalyst component ultra fine particles. It is believed that this wax product contamination is as a result of one or both of: (a) Catalyst support dissolution during aqueous impregnation of the catalyst support with the active catalyst component (during preparation of the catalyst) which may result in precipitation and coating of the bulk support material with a physically bonded amorphous layer of the support material whereon the active catalyst component is deposited. This amorphous layer is insufficiently anchored and results in dislodgement of and washing out of active catalyst component rich ultra fine particles during FT synthesis; and (b) The FT synthesis catalyst is susceptible to hydrothermal attack that is inherent to realistic FT synthesis conditions. Such a hydrothermal attack on exposed and unprotected support material will result in contamination of the waxy hydrocarbon product with ultra fine particular matter rich in the active catalyst component.
WO 99/42214, WO 02/07883, WO 03/012008 and U.S. Pat. No. 7,365,040 all disclose modification of a FT synthesis catalyst support with a modifying component to reduce the dissolution of the catalyst support in aqueous environment, including hydrothermal attack thereby to reduce the negative effect of ultra fine particles rich in active catalyst component contaminating the waxy hydrocarbon product. These documents focus on Si as a modifying component, but a large number of other modifying components such as Zr, Ti, Cu, Zn, Mn, Ba, Co, Ni, Na, K, Ca, Sn, Cr, Fe, Li, Tl, Mg, Sr, Ga, Sb, V, Hf, Th, Ce, Ge, U, Nb, Ta, W and La are also mentioned. The Journal of Catalysis 92, 1-10 (1985) discloses alumina modified with tungsten.
It has now surprisingly been found that when a catalyst support is modified with tungsten instead of silicon, solubility of the support is even further reduced. Even more surprising it has also been found that when the tungsten containing support is calcined at a temperature above 900° C., the solubility of a FT synthesis catalyst or support prepared from the tungsten modified support can be further reduced to even more acceptable levels. It was also unexpectedly found that, in at least some cases, the FT synthesis activity of the catalyst prepared from the tungsten modified support improved compared to a catalyst made from an unmodified support.
When a catalyst support is modified with Si, calcination of the silica containing support prior to impregnation with an active metal component, such as Co, takes place at a temperature of about 500° C. (see WO 99/42214 on page 15 line 9). This temperature is below the calcination temperature set by the invention, i.e. greater than 900° C. The inventors have thus found that when a silica modified support is calcined at temperatures higher than the normal calcination temperature of about 500° C. for calcining such modified supports, the solubility of the modified support calcined at the higher temperatures is higher than the solubility at about 500° C. It was accordingly most surprisingly found that when tungsten is used as a modifying component and the tungsten containing support is then calcined at the higher temperatures described above, the solubility of the tungsten modified catalyst support is reduced compared to the tungsten modified catalyst support calcined at lower temperatures.